Feeling blue is normal: We all have down periods as a result of our day-to-day struggles and major events that challenge us. While sometimes those going through even periods of extreme stress and disappointment may call themselves “depressed,” depression is far more than just feeling sad.

Depression is a very real illness that can significantly interfere with the sufferer’s daily life. Living with depression has been likened to “living in a black hole,” and while sadness is often a major symptom of depression, some sufferers say they don’t feel sad, but rather use words like lifeless, empty, helpless, hopeless or apathetic to describe their moods.

When thinking of depression, we often first think of the psychological toll that depression takes on the sufferer. There may be a withdrawal from social circles, a loss of interest in hobbies or family relationships, an inability to focus at work or sadness or irritability that affects personal relationships. What many people don’t realize, however, is that the symptoms of depression are often physical.

Sadness You Can Feel

While some of the signs of depression are psychological, “most of the signs are physical,” according to Michael Gitlin, professor of clinical psychiatry at the University of California - Los Angeles School of Medicine. These physical symptoms are so common and so closely tied to depression that they are listed among the diagnostic criteria for depression.

There are many theories as to why depression sufferers develop psychosomatic symptoms. However, it is known that depression lowers the pain threshold of sufferers. Further, the regions of the brain that regulate mood also regulate pain. When these regions are affected by depression, sufferers are likely to experience sensations as being more painful than a non-depressed counterpart.